Integrated Circuit with Conductive Line Having Line-Ends

ABSTRACT

A semiconductor device is disclosed, including a plurality of conductive features disposed over a substrate. A dielectric layer separates the conductive features. A conductive line is provided, connecting a subset of the conductive features. The conductive line includes a line-like portion and a line-end portion.

The present application is a continuation application of U.S. application Ser. No. 15/384,076, filed Dec. 19, 2016, which is a divisional application of U.S. application Ser. No. 14/104,917, filed Dec. 12, 2013, each of which is herein incorporated by reference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC design and materials have produced generations of ICs where each generation has smaller and more complex circuits than previous generations. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased.

This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of IC processing and manufacturing. For these advances to be realized, similar developments in IC processing and manufacturing are needed. When a semiconductor device such as a metal-oxide-semiconductor field-effect transistor (MOSFET) is scaled down through various technology nodes, interconnects of conductive lines and associated dielectric materials that facilitate wiring between the transistors and other devices play a more important role in IC performance improvement. Although existing methods of fabricating IC devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart of an example method for fabricating a semiconductor integrated circuit (IC) constructed according to various aspects of the present disclosure.

FIG. 2 is a cross-sectional view of an example semiconductor device precursor according to the method of FIG. 1.

FIGS. 3A and 3B are cross-sectional views of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIG. 4A is a top schematic view of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIGS. 4B and 4C are cross-sectional views of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIGS. 4D and 4E are flowcharts of example methods for fabricating a semiconductor integrated circuit (IC) constructed according to various aspects of the present disclosure.

FIGS. 5A and 5C are top schematic views of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIGS. 5B and 5D are cross-sectional views of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIGS. 5E and 5F are flowcharts of example methods for fabricating a semiconductor integrated circuit (IC) constructed according to various aspects of the present disclosure.

FIGS. 6A and 7A are top schematic views of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIGS. 6B and 7B are cross-sectional views of an example semiconductor integrated circuit (IC) at fabrication stages constructed according to the method of FIG. 1.

FIG. 6C is a flowchart of an example method for fabricating a semiconductor integrated circuit (IC) constructed according to various aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as being “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

FIG. 1 is a flowchart of one embodiment of a method 100 of fabricating one or more semiconductor devices according to aspects of the present disclosure. The method 100 is discussed in detail below, with reference to a semiconductor device 300 shown in FIGS. 2, 3, 4A-4B, 5A-5C, 6 and 7A-7B for the sake of example. It is understood that additional steps can be provided before, during, and after the method, and some of the steps described can be replaced or eliminated for other embodiments of the method.

Referring to FIGS. 1 and 2, the method 100 begins at step 102 by receiving a semiconductor device precursor 200. The semiconductor device precursor 200 includes a substrate 210. In the present embodiment, the substrate 210 includes silicon. In alternative embodiments, the substrate may include germanium, silicon germanium, gallium arsenide or other appropriate semiconductor materials. Alternatively and for some embodiments, the substrate 210 may include an epitaxial layer. For example, the substrate 210 may have an epitaxial layer overlying a bulk semiconductor. Further, the substrate 210 may be strained for performance enhancement. For example, the epitaxial layer may include a semiconductor material different from those of the bulk semiconductor such as a layer of silicon germanium overlying bulk silicon or a layer of silicon overlying a bulk silicon germanium formed by a process including selective epitaxial growth (SEG). Furthermore, the substrate 210 may include a semiconductor-on-insulator (SOI) structure such as a buried dielectric layer. Also alternatively, the substrate 210 may include a buried dielectric layer such as a buried oxide (BOX) layer, such as that formed by a method referred to as separation by implantation of oxygen (SIMOX) technology, wafer bonding, SEG, or other appropriate methods. In fact various embodiments may include any of a variety of substrate structures and materials.

The semiconductor device precursor 200 may also include various isolation features 220. The isolation features 220 separate various device regions in the substrate 210. The isolation features 220 include different structures formed by using different processing technologies. For example, the isolation features 220 may include shallow trench isolation (STI) features. The formation of a STI may include etching a trench in the substrate 210 and filling in the trench with insulator materials such as silicon oxide, silicon nitride, or silicon oxynitride. The filled trench may have a multi-layer structure such as a thermal oxide liner layer with silicon nitride filling the trench. A chemical mechanical polishing (CMP) may be performed to polish back excessive insulator materials and planarize the top surface of the isolation features 220.

The semiconductor device precursor 200 also includes one or more first conductive features 230. In one embodiment, the first conductive feature 230 may include high-k/metal gates (HK/MGs), a three-dimension HK/MGs wrapping over a fin-like structure. As an example, the HK/MGs may include a gate dielectric layer and metal gate (MG). The gate dielectric layer may include LaO, AlO, ZrO, TiO, Ta₂O₅, Y₂O₃, SrTiO₃ (STO), BaTiO₃ (BTO), BaZrO, HfZrO, HfLaO, HfSiO, LaSiO, AlSiO, HfTaO, HfTiO, (Ba,Sr)TiO₃ (BST), Al₂O₃, Si₃N₄, oxynitrides (SiON), or other suitable materials. The MG may include a single layer or multi layers, such as a metal layer, a liner layer, a wetting layer, and an adhesion layer. The MG may include Ti, Ag, Al, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, TiN, TaN, Ru, Mo, Al, WN, Cu, W, or any suitable materials. Additionally, sidewall spacers (not shown) are formed on the sidewalls of the HK/MGs.

In another embodiment, the first conductive features 230 include electrodes, capacitors, resistors or a portion of a resistor. In yet another embodiment, the first conductive features 230 include a portion of the interconnect structure. For example, the first conductive features 230 include contacts, metal vias, or metal lines.

The semiconductor device precursor 200 also includes second conductive features 240 in the substrate 210. A top surface to the second conductive feature 250 may not be at a same horizontal level as a top surface of the first conductive feature 230. In one embodiment, the second conductive features 240 include doped regions (such as sources or drains). In another embodiment, the second conductive features 240 include electrodes, capacitors, resistors or a portion of a resistor, or a portion of the interconnect structure.

The semiconductor device precursor 200 also includes a first dielectric layer 250 deposited over the substrate 210, including between/over each of the first conductive features 230 and over the second conductive features 240. The first dielectric layer 250 includes silicon oxide, silicon nitride, oxynitride, a dielectric material having a dielectric constant (k) lower than thermal silicon oxide (therefore referred to as low-k dielectric material layer), or other suitable dielectric material layer. The first dielectric layer 250 includes a single layer or multiple layers. A CMP may be performed to remove excessive the first dielectric layer 250 to expose the top surface of the first conductive features 230, as well as to provide a substantially planar top surface for the first conductive features 230 and the first dielectric layer 250.

The semiconductor device precursor 200 also includes a second dielectric layer 260 over the first conductive features 230 and the first dielectric layer 250. The second dielectric layer 410 is similar in many respects to the first dielectric layer 250 discussed above.

The semiconductor device precursor 200 also includes conductive plugs 270 in the first and second dielectric layers, 250 and 260, to form full contacts extending down to the first and the second conductive features, 230 and 240. The conductive plugs 270 are formed along a first direction. The conductive plugs 270 may include copper (Cu), aluminum (Al), tungsten (W), copper or copper alloy, such as copper magnesium (CuMn), copper aluminum (CuAl) or copper silicon (CuSi), or other suitable conductive material. In some cases, the conductive plug 270 is referred to as a metal plug.

Referring to FIGS. 1 and 3, once the semiconductor device precursor 200 is received, the method 100 proceeds to step 104 by depositing a third dielectric layer 310 over the second dielectric layer 260, including over the conductive plugs 270, and a hard mask (HM) layer 320 over the third dielectric layer 310. The third dielectric layer 310 is similar in many respects to the first dielectric layer 250 discussed above in association with FIG. 2.

The HM layer 320 includes silicon nitride, silicon oxide, silicon carbide, titanium oxide, titanium nitride, tantalum oxide, tantalum nitride, or any suitable materials. In addition, the HM layer 320 may include a single layer or multiple layers. The HM layer 320 is different from the third dielectric layer 310 to achieve etching selectivity during a subsequent etch, which will be described later. The HM layer 320 may be deposited by suitable techniques, such as CVD, or physical vapor deposition (PVD). In one embodiment, the HM layer 320 includes a stack of layers: a bottom titanium nitride layer deposited over the third dielectric layer 310; an anti-reflection coating (ARC) layer deposited over the bottom titanium nitride layer and a top titanium nitride layer deposited over the ARC layer, as shown in FIG. 3B.

Referring to FIGS. 1 and 4A-4B, the method 100 proceeds to step 106 by forming a first opening 410 on the HM layer 320. The first opening 410 has a first length W₁ landing in a plane, which is perpendicular to the first direction, and a first width W₁. In the present embodiment, the first length W₁ is defined by two line-ends 412 and is substantially larger than the first width W₁. Therefore, in some cases, the first opening 410 is referred to as a line-like opening. The first opening 410 is aligned to a subset of the conductive plugs 270, now labeling them with the reference number 270A. The first opening 410 may be formed by a procedure (shown in FIG. 4D) that includes depositing a resist layer (block 450), forming a first resist pattern on the resist layer, and etching the HM layer 320 using the first resist pattern as an etch mask (block 452). The first resist pattern is then removed by wet stripping or plasma ashing (block 454). In one embodiment, the first openings 410 are formed by removing the top titanium nitride layer of the HM stack 320 using the first resist pattern as an etch mask, as shown in FIG. 4C and FIG. 4E (blocks 460, 462, and 464).

Referring to FIGS. 1 and 5A-5C, the method 100 proceeds to step 108 by forming a second opening 510 on the HM layer 320, landing in the same place as (or near to) where the first opening 410 landed. In the present embodiment, the second opening 510 fully contacts, or overlaps with, the first opening 410 at the line-ends 412. Thus, the second opening 510 forms a new line-end 512 for the first opening 410. In other words, with the second opening 510, the line-end 412 is replaced by the new line-ends 512. The second opening 510 has a second length L₂ along the same direction as the first length W₁ and a second width W₂ along the same direction as the first width W₁. The second length L₂ is substantially smaller than the first length L₁. The second width W₂ is same or larger than the first width W₁. In some cases, the second opening 510 is referred to as a line-end opening.

In one embodiment, the second width W₂ is larger than the first width W₁ and the second opening 510 locates symmetrically to the first opening 410 at the line-end 412 (as shown in FIG. 5A). In another embodiment, the second width W₂ is larger than the first width W₁ and the second opening 510 locates asymmetrically to the first opening 410 at the line-end 412 (as shown in FIG. 5C). In some embodiments, the second opening 510 has a rectangular shape.

As shown in FIG. 5E, the second openings 510 may be formed similar in many respects to the first opening 410 discussed above in association with FIGS. 4A and 4B, which includes forming a resist layer over the HM layer 320 and the first opening 410, forming a second resist pattern on the resist layer (block 550) and etching the HM layer 320 using the second resist pattern as an etch mask (block 552). The second resist pattern is then removed by wet stripping or plasma ashing (block 554). In one embodiment, the second openings 510 are formed by removing the top titanium nitride layer of the HM stack 320 using the second resist pattern as an etch mask, as shown in FIGS. 5D and 5F (blocks 560, 562, and 564).

Referring to FIGS. 1 and 6A-6B, the method 100 proceeds to step 110 by forming a dielectric trench 610 to expose the conductive plugs 270A. In the present embodiment, the dielectric trench 610 is formed by etching the third dielectric layer 310 using the HM layer 320 with the first and the second openings, 410 and 510, as an etch mask. In one embodiment, the etch process selectively removes the third dielectric layer 310 but substantially does not etch the HM layer 320. Therefore the dielectric trench 610 carries a profile of a combination of the first and the second openings, 410 and 510. In another embodiment (shown in FIG. 6C), using the HM layer 320 with the first and the second openings, 410 and 510, in the top titanium nitride layer of the HM stack 320 as an etch mask, the dielectric trench 610 is formed by etching the ARC layer (block 650), the bottom titanium nitride layer (block 652) and the third dielectric layer 310 (block 654).

The dielectric trench 610 has a line-like portion 612 and a line-end portion 614. The line-end portion 614 has the second width W₂, which is equal or larger than the first width W₁ of the line portion 612. In one embodiment, the second width W₂ is larger than the first width W₁ and the line-end portion 614 located symmetrically to the line-like portion 612. In another embodiment, the second width W₂ is larger than the first width W₁ and the line-end portion 614 locates asymmetrically to the line-like portion 612.

Referring to FIGS. 1 and 7A-7B, the method 100 proceeds to step 112 by filling in the dielectric trench with conductive materials and forming a conductive line 710. In one embodiment, a barrier layer is filled in the dielectric trench 610 first by a proper deposition technique, such as PVD and CVD. The barrier layer may include a metal and is electrically conductive but does not permit (or reduces) inter-diffusion and reactions between the third dielectric material layer 310 and the conductive material to be filled in the dielectric trench 610. The barrier layer may include refractory metals and their nitrides. In various examples, the barrier layer includes TiN, TaN, Co, WN, TiSiN, TaSiN, or combinations thereof. The barrier layer may include multiple films.

The conductive material 710 then fills in the dielectric trench 610, as well as over the barrier layer. The conductive material 710 may include copper (Cu), aluminum (Al), tungsten (W), copper or copper alloy, such as copper magnesium (CuMn), copper aluminum (CuAl) or copper silicon (CuSi), or other suitable conductive material. The conductive material 710 may be deposited by y PVD, CVD, metal-organic chemical vapor deposition (MOCVD), or plating. A recess is then performed to etch back the excessive conductive material 710, as well as the excessive barrier layer, to form the conductive line 710 and a substantial planar surface with the third dielectric layer 310. For example, the recess is performed by chemical mechanical polishing (CMP).

Referring again to FIG. 7A, in the present embodiment, the conductive line 710 carries the profile of the dielectric trench 610 and has a line portion 712 and a line-end portion 714. The line-end portion 714 has the second width W₂, which is equal or larger than the first width W₁ of the line portion 712. In one embodiment, the second width W₂ is larger than the first width W₁ and the line-end portion 714 located symmetrically to the line portion 712. In another embodiment, the second width W₂ is larger than the first width W₁ and the line-end portion 714 locates asymmetrically to the line portion 712.

Additional steps can be provided before, during, and after the method 100, and some of the steps described can be replaced, eliminated, or moved around for additional embodiments of the method 100. The device 300 may undergo further CMOS or MOS technology processing to form various features and regions.

Based on the above, the present disclosure offers a semiconductor device. The semiconductor device may include a plurality of conductive features over a substrate, a dielectric layer separating the conductive features, and a conductive line connecting a subset of the conductive features. The conductive line may include a line-like portion having a first length and a first width and a line-end portion connecting to an end of the line-like portion, wherein the line-end portion has a second width, equal to or larger than the first width, wherein the line-end portion has a second length, smaller than the first length.

In some embodiments, the line-end portion of the conductive line stays symmetrically to the line-like portion. The line-end portion of the conductive line, having a larger width than the width of the line-like portion, may stay asymmetrically to the line-like portion. The line-end portion may have a rectangular shape. The plurality of conductive features may extend through the dielectric layer. The semiconductor device may include a hard mask (HM) layer disposed over the dielectric layer, wherein the conductive line is formed within the dielectric layer and the HM layer.

In other embodiment, a semiconductor device is provided, which may include a substrate, a first set of conductive features formed over the substrate, a second set of conductive features formed over the substrate, a first dielectric layer separating the first set of conductive features and the second set of conductive features, a second dielectric layer formed over the first dielectric layer, and a hard mask (HM) layer formed over the second dielectric layer, wherein the HM layer and the second dielectric layer comprise a line-end opening and a line-like opening, wherein the line-end opening is aligned to the first set of conductive features, wherein the line-end opening connects with the line-like opening at a first end of the line-like opening, wherein the line-like opening has a first width and the line-end opening has a second width greater than the first width.

In some embodiments, a conductive line is provided connecting the first set of conductive features and the second set of conductive features. The conductive line may be disposed in a trench within the second dielectric layer. The line-like opening may be aligned to the second set of conductive features. The line-end opening may have a rectangular shape. A second line-end opening may be provided, connected with the line-like opening at a second end of the line-like opening. In some embodiments, the HM layer includes a stack of layers, including a bottom titanium nitride layer, an anti-reflection coating (ARC) layer formed over the bottom titanium nitride layer, and a top titanium nitride layer formed over the ARC layer.

In another embodiment, a semiconductor device is provided, including a substrate, a first dielectric layer formed over the substrate, a second dielectric layer formed over the first dielectric layer, a set of conductive features extending through the first and second dielectric layers, a third dielectric layer formed over the second dielectric layer, and a hard mask (HM) layer formed over the third dielectric layer, wherein a trench is formed within the third dielectric layer and the HM layer, the trench comprising a line-like opening and two line-end openings connecting with the line-like opening at respective ends of the line-like opening, wherein the two line-end openings are aligned to a first subset of the set of conductive features, wherein the line-like opening has a first width and the two line-end openings each have a second width greater than the first width.

In some embodiments, the trench is filled with conductive material to form a conductive line. The conductive line may connect at least two conductive features within the set of conductive features. The line-like opening may be aligned to a second subset of the set of conductive features. A length and a width of each line-end opening may be the same. The line-like opening may have a first length and the line-end openings each may have a second length smaller than the first length. In some embodiments, the HM layer includes a stack of layers, including a bottom titanium nitride layer, an anti-reflection coating (ARC) layer formed over the bottom titanium nitride layer, and a top titanium nitride layer formed over the ARC layer.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A device comprising: a first conductive feature disposed over a substrate; a first dielectric layer surrounding the first conductive feature; and a conductive line electrically coupled to the first conductive feature, the conductive line including: a line-like portion having a first length and a first width; and a first line-end portion connecting to a first end of the line-like portion and disposed over the first conductive feature, wherein the first line-end portion has a second width that is larger than the first width, wherein the first line-end portion has a second length that is smaller than the first length.
 2. The device of claim 1, wherein the conductive line further includes a second line-end portion connecting to a second end of the line-like portion, wherein the second line-end portion has a third width that is larger than the first width, wherein the second line-end portion has a third length that is smaller than the first length.
 3. The device of claim 2, wherein the third width is the same as the second width.
 4. The device of claim 2, wherein the first end of the line-like portion is opposite the second end of the line-like portion.
 5. The device of claim 2, further comprising a second conductive feature disposed over the substrate, and wherein the first line-end portion physically contacts the first conductive feature and the second line-end portion physically contacts the second conductive feature.
 6. The device of claim 5, wherein at least one of the first conductive feature and the second conductive feature is a contact.
 7. The device of claim 6, further comprising a doped region disposed in the substrate, and wherein the contact extends to the doped region.
 8. A device comprising: a first dielectric layer disposed over a substrate; a first conductive feature and a second conductive feature extending through the first dielectric layer; and a conductive line physically contacting the first conductive feature, the conductive line including: a line-like portion having a first length and a first width; a first line-end portion connecting to a first end of the line-like portion and disposed over the first conductive feature, wherein the first line-end portion has a second width that is larger than the first width, wherein the first line-end portion has a second length that is smaller than the first length; and a second line-end portion connecting to a second end of the line-like portion, wherein the second line-end portion has a third width that is larger than the first width, wherein the second line-end portion has a third length that is smaller than the first length.
 9. The device of claim 8, further comprising; a gate stack disposed over the substrate; and a doped feature disposed in the substrate adjacent the gate stack, and wherein the first conductive feature extends to the gate stack and the second conductive feature extends to the doped feature.
 10. The device of claim 8, wherein the second line-end portion is disposed over the second conductive feature.
 11. The device of claim 8, further comprising a second dielectric disposed over the second conductive feature thereby preventing the conductive line from physically contacting the second conductive feature.
 12. The device of claim 8, wherein the first line-end portion physically contacts a top surface of the first conductive feature, the top surface of the first conductive feature facing away from the substrate, and wherein the second line-end portion physically contacts a top surface of the second conductive feature, the top surface of the second conductive feature facing away from the substrate.
 13. The device of claim 8 wherein the conductive line is formed of a metal containing material and a barrier layer material at least partially surrounding the metal containing material.
 14. The device of claim 8, wherein the second width is equal to the third width, and wherein the second length is equal to the third length.
 15. A device comprising: a first dielectric layer disposed over a semiconductor substrate; a second dielectric layer disposed over the first dielectric layer; a first conductive feature extending through the first dielectric layer and the second dielectric layer; and a conductive line disposed over the first conductive feature, the conductive line including: a line-like portion having a first length and a first width; and a first line-end portion connecting to a first end of the line-like portion and physically contacting the first conductive feature, wherein the first line-end portion has a second width that is larger than the first width, wherein the first line-end portion has a second length that is smaller than the first length.
 16. The device of claim 15, wherein the first conductive feature is a contact extending through the first dielectric layer and the second dielectric layer to at least one of a gate stack and a source/drain feature.
 17. The device of claim 15, further comprising a second line-end portion (714) connecting to a second end of the line-like portion, wherein the second line-end portion has a third width that is larger than the first width, wherein the second line-end portion has a third length that is smaller than the first length.
 18. The device of claim 17, wherein the first line-end portion of the conductive line is centered with respect to the first end of the line-like portion, and wherein the second line-end portion of the conductive line is centered with respect to the second end of the line-like portion.
 19. The device of claim 17, wherein the first line-end portion of the conductive line is not centered with respect to the first end of the line-like portion, and wherein the second line-end portion of the conductive line is not centered with respect to the second end of the line-like portion.
 20. The device of claim 17, further comprising a second conductive feature extending through the first dielectric layer and the second dielectric layer, the second conductive feature physically contacting the line-like portion of the conductive line. 